This invention relates to a measuring apparatus which measures the polarizing activity and the distribution thereof of a test substance with a high resolution, by detecting a beam which has interacted with a tiny area of substance at the tip of probe, and by utilizing the polarization characteristics the beam presents.
It becomes important to observe, for various test samples, the distribution of their optical activities (circular dichroism and optical rotation) at very tiny areas, and to obtain quantitative evaluations of those optical activities. Such optical activities include natural optical activities, electro-optical activities, magneto-optical activities, and piezo-optical activities, and with the recent technical advance in the field of memories having a gigantic capacity such as a hard disk, opto-magnetic disk, etc., a demand for the equipment allowing a precise observation and measurement of magneto-optical effects becomes rapidly intense.
For example, to observe the distribution of magneto-optic effects as one aspect of such optical activities with a high precision requires to observe magnetic sectors and barriers, and the well-known method used for this purpose includes polarized light microscopy, Lorenz transmission electronmicroscopy, spin-polarized scanning electronmicroscopy, and magnetic force microscopy. A recent article reports an observation of the magnetic barriers of a vertically magnetized membrane by the use of a scanning near field optical microscope (APPLIED OPTICS, Vol. 31, No. 22, 1992, p. 4563, E. Betzig et al.).
Here a scanning near field optical microscope will be briefly sketched. A widely available method consists of sharpening an optic fiber or a beam transmitting body, and preparing a minute opening at its tip having a diameter equal to or less than the wavelength of beam. By the same method with which a conventional scanning atomic force microscope or a scanning tunnel microscope adjusts the distance between a cantilever and a sample, the minute opening is placed so close to the surface of a sample that the distance in between is equal to or less than the wavelength of beam. By introducing, while maintaining above state, a beam into the optic fiber with such minute opening, radiating a tiny area of a sample with the beam emanating from the minute opening, and scanning the beam over the sample in a two-dimensional plane, a microscope can achieve a high resolution microscopy in accordance with the size of minute opening. In the example mentioned earlier where a scanning near field optical microscope was used for the observation of magnetic barriers, a linearly polarized beam emanating from a minute opening is allowed to radiate a sample, and the beam transmitting through the sample is received by an analyzer (cross-Nichol method).
On the other hand, the method by which to quantitatively determine the circular dichroism or optical rotation of a sample, for example, on the basis of magneto-optical effects (methods dependent on other optical activities works on the essentially same principle) is described in detail in "Light and magnetism" published by Asakura Publishing Co. (written by Sato, K.). The optical rotation due to magnetism can be determined by perpendicularly intersecting polarizers (cross-Nichol method), Faraday-cell method, and a rotational analyzer. The use of a quarter-wave plate will allow the measurement of the circular dichroism of sample. Further, modulation of a circularly polarized beam (circular polarization modulation) will enable the measurement of both the magneto-optical rotation and magneto-optical circular dichroism with a high sensitivity.
Here, circular polarization modulation will be briefly sketched with reference to FIG. 2. A linearly polarized beam having passed through a linear polarizer 101 is given, by a piezo-optical modulator 102 working on birefringence, an optical delay which changes at a frequency of p (Hz). Then, the same beam, after having been reflected from or passed through a sample 103 (the beam is reflected from a sample in FIG. 2), is allowed to pass through an analyzer 104 to reach a light receiving element 105 for registration. From p (Hz) component and 2p (Hz) component of the beam having passed through the analyzer 104, it is possible to determine the circular dichroism and optical rotation the beam has undergone, respectively.
The principle underlying circular polarization modulation will be described by equations. For brevity, the direction along which a beam transmits is supposed to coincide with z-axis. Let's assume that in FIG. 2 the linear polarizer 101 has an angle of 45.degree. with respect to x-axis. The electric field E.sub.1 of the beam having passed through the linear polarizer 101 can be expressed by: EQU E.sub.1.sup..varies. (i+j) (1)
given that i and j are the unit vectors of x- and y-axes respectively.
Given that there is a delay of .delta. between x- and y-components of the electric field E.sub.2 of the beam which has passed through the piezo-optical modulator 102, EQU E.sub.2.sup..varies. {i+exp(i.delta.)j} (2).
Assumed that the unit vectors of right- and left-circularly polarized beams are expressed by following equations respectively: EQU r=(i+ij)/2.sup.1/2 EQU I=(i-ij)/2.sup.1/2,
then, E2 can be expressed by the following equation: EQU E2.sup..varies. {(1-i.exp(i.delta.))r+(1+i.exp(i.delta.))i}(3)
Suppose that the complexly expressed amplitude reflections of right- and left-circulatory polarized beams are expressed by r+exp(i.theta.+) and r-exp(i.theta.-) respectively, then the electric field E.sub.3 of reflected beam can be expressed by: EQU E.sub.3.sup..varies. ((1-i.exp(i.delta.))r+exp(i.theta.+)r+(1+i.exp(i.delta.))r.exp(i.theta.-)i }(4).
The intensity I of the beam emanating from the analyzer having an angle of .phi. with respect to x-axis is expressed by: EQU I.sup..varies. {R+(.DELTA.R/2) sin .delta.+R sin (.DELTA..theta.+2.phi.) cos .delta.} (5)
where
R=(r+2+r-2)/2 PA1 .DELTA.R=r+2-r-2 PA1 .DELTA..theta.=.theta.+-.theta.-=-2.theta..sub.k PA1 .DELTA.R/R=4.eta..sub.k
and where .theta..sub.k represents a Kerr's rotation angle and .eta..sub.k a Kerr's ellipticity. Assumed that .phi.=0, and .DELTA..theta. is sufficiently small, .delta..about.sin 2.pi.pt. Then, the equation can be resolved by the use of Bessel function into: EQU I.about.I(0)+I(p) sin 2.pi.pt+I(2p) cos 4.pi.pt+ (6).
In this equation, I(o), I(p), and I(2p) represent factors respectively containing 0th-order, 1st-order and 2nd order Bessel functions, and EQU I(p).sup..varies. .eta..sub.k, I(2p).sup..varies. .theta..sub.k(7).
Therefore, p(Hz) component gives the Kerr's ellipticity and 2p(Hz) gives the Kerr's rotation angle. For details, see the above-described "Light and magnetism."
The above-described various methods employed for the observation of minute magnetic sectors have a number of problems as will be described later. For example, polarized light microscopy, operating in the same manner as conventional optical microscopy, has its resolution restricted by the diffraction limit of a beam used, and only achieves a resolution that allows distinguishing the width of about half the wavelength of beam used. Further, as it depends on the cross-Nichols method for detecting the optical activities of a sample, its detection sensitivity is low. Lorenz transmission electronmicroscopy has a resolution sufficiently high to distinguish about 10 nm intervals, but it is only applied to a thinly sectioned sample. Spin-polarized scanning electronmicroscopy has a problem in that it requires a large cost for installment. Magnetic force microscopy has a considerably high resolution that allows discrimination of several tens nm intervals, but it can be scarcely applied for the quantitative determination of the magnitude of a magnetic field or magnetization. Scanning near field optical microscope has its resolution determined principally by the diameter of opening of the probe, and has a considerably high resolution. The conventional minute spot scanning microscopy, however, usually depends, for the detection of optical activities of a sample, on the cross Nichols method, and presents following problems. It allows only a low sensitivity. Notwithstanding that the closer the minute spot beam emanating from a minute opening is to a linearly polarized beam, the higher the detection sensitivity, the minute spot beam emanating from a minute opening is usually elliptically polarized. This may form another cause for a lowered sensitivity.
Among the apparatuses for quantifying various magneto-optical effects, there are some that allow the very sensitive quantification of a rotation angle through modulation, for example, by the use of a rotating analyzer. This method, however, can not be applied to a tiny area exceeding the typical level handled by a conventional optical microscope.
As illustrated above by referring to the microscopic observation of magneto-optical effects as an example, the conventional methods whereby the distribution and quantification of optical activities of a sample have been obtained have more or less defects to be corrected, although some are advantageous in sensitivity, resolution and tolerance of sample handling, and others are advantageous in cost. What is mentioned above applies to the measurement not only of magneto-optical effects but also of optical activities at large. In view of this the object of the present invention is to provide an apparatus with which it is possible to observe/measure the optical activities of a sample with a high resolution and sensitivity, at a low cost, quantitatively, and without imposing any restrictions on the handling of sample.